Increasingly stringent demands are being placed on the pulp industry to decrease the use of chlorine during bleaching. Permitted discharges of organic chlorine compounds (AOX) in the effluent water from the bleaching plant have been gradually decreased and are now at such a low level that pulp works have in many cases stopped using chlorine gas. The use of chlorine dioxide is also being called into question. The demands made by the environmental authorities in certain countries are so severe that it is difficult to comply with them even if only chlorine dioxide is utilized for bleaching. In addition, consumers have begun to demand paper products which have been bleached entirely without using either chlorine gas or chlorine dioxide, i.e. by so-called TCF (total chlorine free) bleaching.
The pulp industry is therefore seeking methods which permit bleaching of pulp without using these chemicals. A method of this type which has been developed (see SE-A-8902058) involves the unbleached pulp first being delignified with oxygen and then, after washing, being treated with EDTA or another suitable chelating agent in order to remove heavy metals which are bound in the pulp. After the EDTA stage (Q) there follows an intensive peroxide bleaching stage (P), i.e. using hydrogen peroxide. The charge of hydrogen peroxide (H2O2) is relatively high, being 15-35 kg per ton of pulp, depending on the desired brightness and on the bleachability of the pulp. The time required is rather long, being 4 hours or more, and the temperature high, being 80-90xc2x0 C. The term xe2x80x9cstagexe2x80x9d includes a wash in accordance with the TAPPI standard.
A prerequisite for achieving high brightnesses while consuming moderate amounts of bleaching agent is that the pulp, prior to the bleaching, should have been delignified to low kappa numbers, preferably to lower than a kappa number of 16. Normally, taking delignification in the digester house and oxygen delignification too far results in impairment of quality, in particular loss of fibre strength. Nevertheless, in order to achieve a brightness of 85-90% ISO, as required by the market, together with acceptable strength, it is necessary, in order to be able to carry out a chlorine-free bleaching process, that the pulp be produced by a pulping process which yields a low kappa number, less than 20 and preferably less than 15, and a viscosity of at least about 1,000 dm3/kg. This process should preferably contain an oxygen gas delignification stage. However, using the modified cooking methods which have been developed in recent years, it has been found possible to achieve very low kappa numbers without loss of strength. For example, it is possible, using a modification of Kamyr""s is continuous cooking process MCC (modified continuous cooking) combined with MC oxygen delignification, to get down to and below a kappa number of 10 with soft wood and a kappa number of 8 with hard wood while retaining strength properties. If the ITC (isothermal cooking) process, for which Kamyr is seeking a patent, is used as well, even lower kappa numbers can be obtained; kappa numbers of less than 15 after the digester, giving less than 10 after oxygen delignification, are readily achieved for soft wood.
The modification according to the ITC process involves the hi-heat washing zone in the lower part of the continuous digester also being utilized for countercurrent cooking (see SE-A-9203462). This is brought about by heating to full cooking temperature in the hi-heat circulation and by adding alkaline cooking liquid to this same circulation. The total cooking time in countercurrent is thereby extended to 3-4 hours as compared with about 1 hour in the case of conventional MCC. This results in a very low lignin concentration being obtained at the end of the cooking, in turn providing improved selectivity in the delignification, i.e. the lignin of the wood is efficiently released without the cellulose being attacked to any appreciable extent. The cooking and the oxygen delignification can thereby be pursued down to very low kappa numbers without impairing the properties of the pulp, ensuring that bleaching with chemicals of the peroxide type and the like can be used for bleaching up to full brightness while retaining acceptable pulp properties.
The relatively high costs associated with using bleaching chemicals, for example peroxide, which do not contain chlorine represent a general problem in connection with chlorine-free bleaching.
The object of the present invention is to produce a method of bleaching chemical paper pulp without using chlorine-containing agents, which method involves the use of peroxide, the peroxide being used in as efficient a manner as possible with a view to being able to achieve, at relatively low cost, a finished bleached pulp of a brightness as required by the market.
Somewhat surprisingly, it has been possible to ascertain, in experiments carried out by Kamyr, that delignification with the aid of peroxide can be carried out using very low charges essentially without any loss as regards the strength properties of the fibres, i.e. almost without any decrease in viscosity. We have found that we can achieve a delignification of more than 35%, in association with a peroxide consumption of less than 5 kilo/BDTM, without any real decrease in viscosity. This must be considered to be very surprising in view of the results which are presented in SE-A-8902058 (Eka Nobel), for example.
In the enclosed diagrams, which are based on some 100 experiments using QP bleaching, with different charges being used and consumption and kappa number, inter alia, being measured, it has been possible to establish, firstly, that nearly all the delignification is achieved with a peroxide consumption of less than 10 kilo, and that a consumption of less than 5 kilo was already sufficient to achieve a major part of the delignification. This in itself is remarkable. In addition, it was possible to establish that this delignification (kappa reduction) with relatively low peroxide charges can be carried out essentially without loss of the strength properties of the fibre, something which, taken as a whole, truly is remarkable. Thus, Kamyr AB, with the aid of the said experiments, has been able to establish that, above a certain consumption, exceeding about 10 kilo/BDTM, which is relatively low, the ability of the peroxide to delignify declines, in principle to zero. This implies that peroxide which is added over and above that, and which is consumed, does not delignify but instead bleaches remaining lignins and attacks the carbohydrates; the consequences of this are that the fibres are attacked and the strength properties are thereby diminished, and that there is a risk of the bleached pulp subsequently yellowing owing to the remaining lignin content.
Using these observations, Kamyr has concluded that, in connection with peroxide bleaching, a first peroxide bleaching stage (preferably after Q) should be used in which the peroxide charge is relatively low and that this peroxide stage should be followed by a delignifying stage, for example using ozone, as a result of which the kappa number is preferably brought below 4, but preferably below 3 and most preferably 1 or less, and that the latter delignifying stage should be followed by an essentially purely bleaching peroxide stage using a higher charge of peroxide. By these means, the peroxide consumption is optimized so that a fully bleached TCF pulp of high quality can be obtained at low cost.
The present object is achieved by a method for bleaching chemical paper pulp, which has been cooked and preferably oxygen-delignified, using methods which preserve viscosity and strength, to low kappa numbers, especially lower than 16, but preferably lower than 10, without employing chlorine-containing chemicals, using a bleaching sequence containing at least 3 stages, whose first stage is a P stage, preferably preceded by a Q stage, characterized by a first P stage which is an essentially delignifying P stage, the charge of peroxide being less than 12 kilo/BDMT, and by a delignifying, acid stage, following the said P stage and preferably including a wash, as well as by a second, preferably alkaline, P stage, following the said acid stage and preferably including a wash, which second P stage is an essentially bleaching peroxide stage, the peroxide charge exceeding 3 kilo/BDMT and exceeding the quantity of peroxide which was added in the said first P stage.
In this context, ozone is an interesting chemical for use in the intermediate delignifying stage, the so-called acid stage. It has been found that the use of an ozone bleaching stage (Z) appreciably decreases the lignin content, i.e. reduces the kappa number. This is important, since a pulp which has been delignified and bleached using only peroxide or oxygen/peroxide still contains a relatively high content of lignin, which accordingly affects the subsequent yellowing tendency of the pulp. Under these circumstances, the pulp yellows when heated or when irradiated with sunlight. Ozone thus removes further lignin, thereby making the brightness of the pulp more stable.
According to a further aspect of the invention, the process is improved by the charge of peroxide in the said first P stage being between 3 and 7 kilo/BDMT, preferably being between 4-6 kilo/BDMT and more preferably about 5 kilo/BDMT.
According to a further aspect of the invention, the process is improved by the charge of peroxide in the said second P stage exceeding 3 kilo/BDMT, preferably exceeding 7 kilo/BDMT, preferably being less than 25 kilo/BDMT, and more preferably being between 11 to 20 kilo/BDMT.
According to a further aspect of the invention, the process is improved by the filtrate from the said second P stage being conveyed to the said first P stage.
The process according to the invention is first and foremost intended for pulp of average consistency, i.e. having a pulp consistency between 5-25%.
According to a further aspect of the invention, the process is improved by the said acid stage being an ozone stage.
According to a further aspect of the invention, a preferred embodiment of a TCF bleaching plant is shown.
FIGS. 1-5 show the relationship between kappa number and kappa reduction with peroxide consumption.
FIGS. 6-8 show the relationship between brightness and viscosity with peroxide consumption.
FIGS. 9 and 10 show apparatus for TCF bleaching plants.
In connection with the description below, reference is made to:
FIG. 1, which shows the relationship between the kappa number and the peroxide consumption for hard wood pulp, from which it can be seen that no real kappa reduction is achieved with a peroxide consumption exceeding 7 kilo/BDTM.
FIG. 2, which shows a diagram of the relationship between kappa reduction and the peroxide consumption for hard wood pulp, from which it can be seen that approximately a good 60% of the kappa reduction was brought about with a consumption of 5 kilo/BDTM, that more than 90% of the kappa reduction was obtained with a consumption of 10 kilo/BDTM, and that no further real kappa reduction is produced by peroxide consumptions exceeding 15 kilo/BDTM.
FIG. 3, which shows a diagram of the relationship between the kappa number and the peroxide consumption for soft wood, from which it can be seen that no further real decrease in the kappa number is produced by peroxide consumptions exceeding 10 kilo.
FIG. 4, which shows a diagram of the relationship between kappa reduction and peroxide consumption for soft wood pulp, from which it can be seen that about 50% of the reduction was achieved with a consumption of about 5 kilo/BDTM, that about 70%. of the kappa reduction was achieved with a consumption of about 10 kilo, and that about 85% of the kappa reduction was achieved with a consumption of about 15 kilo.
FIG. 5, which shows the relationship between kappa number and peroxide consumption for soft wood pulp, on the one hand in association with low peroxide charge in accordance with the invention, and, on the other, in association with conventional high peroxide charging, from which it can be seen that above a certain level of peroxide consumption no real kappa reduction is produced.
FIG. 6, which shows a diagram of the relationship between brightness and peroxide consumption, on the one hand in association with low peroxide charging in accordance with the invention and, on the other, in association with conventional high peroxide charging, from which it can be seen that the increased charging has a brightness-increasing effect. It is thus evident that the high charges of peroxide only give rise to an increase in brightness and not to any further delignification.
FIG. 7, which shows a diagram of the relationship between viscosity decrease and brightness for soft wood pulp, on the one hand in association with peroxide charging according to the invention and, on the other, in association with conventional high peroxide charging, from which it can be seen that, in contrast to the conventional technique, charging according to the invention has no real effect in lowering viscosity.
FIG. 8, which shows a diagram of the relationship between brightness and peroxide charging for hard wood pulp, in association with a three-stage sequence in accordance with the invention, from which it can be seen that a pulp having a very good brightness can be produced using a very low charge of peroxide, and thus at low bleaching cost.
FIG. 9, which shows an exemplifying embodiment with regard to an apparatus array in a TCF bleaching plant according to the invention, and
FIG. 10, which shows a preferred embodiment with regard to an apparatus array for a TCF bleaching plant.